Fuel cells for combining hydrogen and oxygen to produce electricity are well known. A known class of fuel cells includes a solid-oxide electrolyte layer through which oxygen anions migrate; such fuel cells are referred to in the art as “solid-oxide” fuel cells (SOFCs).
In some applications, for example, as an auxiliary power unit (APU) for a transportation application, an SOFC is preferably fueled by “reformate” gas, which is the effluent from a catalytic liquid or gaseous hydrocarbon oxidizing reformer, also referred to herein as “fuel gas”. Reformate typically includes amounts of carbon monoxide (CO) as fuel in addition to molecular hydrogen. The reforming operation and the fuel cell operation may be considered as first and second oxidative steps of the hydrocarbon fuel, resulting ultimately in water and carbon dioxide. Both reactions are preferably carried out at relatively high temperatures, for example, in the range of 700° C. to 1000° C.
A complete fuel cell stack assembly includes fuel cell subassemblies and a plurality of components known in the art as interconnects, which electrically connect the individual fuel cell subassemblies in series. Typically, the interconnects include a conductive foam or weave disposed in the fuel gas and air flow spaces adjacent the anodes and cathodes of the subassemblies.
In the prior art, a fuel cell stack is assembled typically by laying up the interconnects and the fuel cell subassemblies in a jig, forming repetitive fuel cell units. Typically, a fuel cell subassembly comprises a ceramic solid-oxide electrolyte layer and a cathode layer coated onto a relatively thick, structurally-significant anode element. In such a prior art assembly, each of the elements in the stack, including the fuel cell subassemblies, becomes a structural and load-bearing element of the stack.
This assembly process and final product are subject to several serious shortcomings. First, assembly is very time-consuming and labor intensive, and thus is expensive. Second, the fuel cell subassembly is relatively fragile and is easily damaged during stack assembly; however, damaged subassemblies cannot be detected and replaced until the entire stack has been assembled, resulting in very time-consuming and expensive rework procedures or scrapping defective assemblies. Third, the fuel cell subassembly is not structurally competent at operating temperatures and thus the stack is dimensionally and structurally unstable. Fourth, the individual elements, and especially the interconnects, are relatively thick, resulting in an undesirably large package for a complete assembly.
What is needed in the art is a means for assembling each fuel cell subassembly into a working configuration such that it can be functionally tested prior to final assembly into a fuel cell stack.
What is further needed in the art is a means for removing a fuel cell subassembly from the load-bearing structure of a fuel cell stack.
What is still further needed in the art is a means for reducing the thickness of each fuel cell repetitive unit in a fuel cell stack.
It is a principal object of the present invention to modularize the structure of a fuel cell stack, and thereby permit functional testing of each module prior to assembly into the stack; to remove the fuel cell subassembly from the load-bearing structure of the stack; to reduce the thickness of each repetitive unit in the stack; and to reduce the cost, difficulty, and complexity of mass-manufacturing fuel cell stack assemblies.